It is known that wobble can be corrected by double reflection from the active facet; see Meeussen et al, U.S. Pat. No. 8,897,132. But all previous embodiments of this concept have bow, which makes them unworkable. In copending application Ser. No. 876,648, a polygon system for correcting wobble by double reflection from the active facet, without bow, is disclosed. In this application, the angles of incidence at the first and second reflection from the facet are recognized as important to the effects of bow, and bow can be almost eliminated by the proper relation between these angles. Bow can be, for all practical purposes, entirely eliminated when the facets have a draft angle, which has a predetermined relation to the angles of incidence at the first and second reflection.
This concept corrects for wobble exactly, at the center of scan. But at the edge of scan, wobble correction is not exact, and there is some residual effect.
Since the beam is offset in the cross-scan plane, as the polygon rotates, the reflected scanning beam after the first reflection will not be in a flat plane. It will lie on the surface of a cone, the apex angle of which is dependent on the offset angle. If the offset angle is changed by a facet tilt, the apex angle of the cone surface containing the reflected ray will change, and the diameter of the cone at some distance from the first reflection will change.
After the second reflection, the curvature of the cone surface is cancelled, and the beam scan will be in a flat plane. This is the principle of bow correction of the concept set forth in said copending application. It can be thought of as the cone being opened up and laid on a flat surface. But if the facet has a tilt angle, the flattened cone will not be the same size as the flattened nominal cone. This means that the scan length, for the same polygon rotation angle, is different for a facet with a tilt angle than for a facet without a tilt angle. The difference is "jitter".
The start of scan detector will start the scan clock when the beam crosses it. Thus, at the beginning of the scan, information will be contiguous with the same information of adjacent scans, whether or not the facet has tilt. But at the end of scan, if the facet has a tilt angle, the information will not be in synchronization with information of adjacent scans.
Jitter can be changed in the concept disclosed in Ser. No. 876,648 by changing the angle of incidence at the first reflection from the facet. But any reduction of jitter is invariably accomplished by an increase in residual wobble at the end of scan.
For some low performance applications of this technique, particularly if the scan length at the photoreceptor is 8 inches, jitter and wobble arewithin specifications. But if the scan length is increased, either jitter or wobble will exceed specifications.
According to the present invention, the start of scan detector is positioned about an axis through the nominal scan line. Thus, if tilt is present in the rotating polygon system and jitter is generated, the angle of the split detector moves the jitter to the beginning and end of scan, leaving the center of scan free of the effects of jitter. The angle of the split detector offset is in approximate relation to the angle of incidence at the first reflection on the rotating polygon.